Convert channel counts to size_t.

IIRC, this was originally requested by ajm during review of the other size_t conversions I did over the past year, and I agreed it made sense, but wanted to do it separately since those changes were already gargantuan.

BUG=chromium:81439
TEST=none
R=henrik.lundin@webrtc.org, henrika@webrtc.org, kjellander@webrtc.org, minyue@webrtc.org, perkj@webrtc.org, solenberg@webrtc.org, stefan@webrtc.org, tina.legrand@webrtc.org

Review URL: https://codereview.webrtc.org/1316523002 .

Cr-Commit-Position: refs/heads/master@{#11229}
diff --git a/webrtc/modules/audio_coding/acm2/acm_resampler.cc b/webrtc/modules/audio_coding/acm2/acm_resampler.cc
index d7ceb8a..dfc3ef7 100644
--- a/webrtc/modules/audio_coding/acm2/acm_resampler.cc
+++ b/webrtc/modules/audio_coding/acm2/acm_resampler.cc
@@ -28,10 +28,10 @@
 int ACMResampler::Resample10Msec(const int16_t* in_audio,
                                  int in_freq_hz,
                                  int out_freq_hz,
-                                 int num_audio_channels,
+                                 size_t num_audio_channels,
                                  size_t out_capacity_samples,
                                  int16_t* out_audio) {
-  size_t in_length = static_cast<size_t>(in_freq_hz * num_audio_channels / 100);
+  size_t in_length = in_freq_hz * num_audio_channels / 100;
   if (in_freq_hz == out_freq_hz) {
     if (out_capacity_samples < in_length) {
       assert(false);
@@ -56,7 +56,7 @@
     return -1;
   }
 
-  return out_length / num_audio_channels;
+  return static_cast<int>(out_length / num_audio_channels);
 }
 
 }  // namespace acm2